1. Field of the Invention
The present invention relates to an ion generating device.
2. Description of the Related Art
A conventional ion generating device generates ions used as a source of corona ions for forming electrostatic latent images in an electrophotography recording machine such as a laser beam printer, copy machine and so on.
Japanese Laid Open Patent Application No. 8-82980 discloses an ion generator having a pair of electrodes formed on a ceramic substrate for generating corona discharge. FIG. 9 is a schematic drawing showing this ion generating device adapted for an electrophotography recording machine.
The electrodes of the ion generating device include a first electrode 102 and a second electrode 104. The first electrode 102 is formed on the substrate 101 by a forming method of a conventional thin film. A dielectric layer 103 coats the first electrode 102, and the surface of the substrate 101 insulates between the electrodes. The second electrode 104 is on the dielectric layer 103, but the second electrode 104 is not located directly above the first electrode 102. A protective layer 108 coats the whole surfaces of the dielectric layer 103 and the second electrode 104 for protection.
As shown in FIG. 9, a conductive drum which functions as a photosensitive medium 105 is near the ion generating device so that the gap formed between the medium and the ion generating device has a predetermined width. The photosensitive medium 105 is positively charged by a DC power source 107.
When a high frequency power source 106 applies high frequency voltage between the first electrode 102 and the second electrode 104, an alternating leakage electric field E having a periodically alternating polarity is generated between the first electrode 102 and the second electrode 104 as indicated by arrows in FIG. 9. The leakage electric field E which penetrates through the atmosphere ionizes gas existing around the surface of the device, such as water vapor and methane gas, and negative and positive ions are generated. The amount of ion generation is proportional to the intensity of the leakage electric field E which penetrates through the atmosphere.
Negative ions generated near the device selectively flow toward the photosensitive medium 105 supplied with a positive bias. Accordingly, the photosensitive medium 105 is uniformly charged with positive potential.
The charged potential on the photosensitive medium 105 can be altered by adjusting the bias voltage applied between the second electrode 104 and the photosensitive medium 105.
However, the ion generating device, which has a protective layer 108 coating the whole surface, has the following problems. A part of the protective layer 108 functions as a protector against collisions with ions. The other parts of the layer 108 which is appears as a stray capacitance formed between the electrodes, are not necessary to protect the electrode. In the ion generating device, the leakage electric field E is weakened by the other parts of the protective layer 108, because the leakage electric field E passes through the other parts of the layer while the field penetrates the atmosphere.
Furthermore, a thicker protective layer is preferable to protect the electrode. However, the thicker protective layer increases the stray capacitance around the electrode. Since the stray capacitance reduces the intensity of leakage electric field, the amount of ions generated by the device decreases. When a high voltage is applied to the device, the thicker protective layer loses effectiveness because the ion collisions will be strengthened. Accordingly, it is difficult to protect the electrode while obtaining sufficient ions.